The present invention pertains in general to the speed control of induction-type motors. More particularly, this invention pertains to a speed control technique that permits operation over a wide speed range using variable duty cycle square wave excitation of the motor windings. The principles of this invention are applicable to multi-phase induction motors and also single phase induction motors. This invention also pertains to an improved initialization circuit which controls the high DC voltage applied to one or more high voltage switches which in turn control the current to the motor windings. The initialization circuit may comprise an AC to DC converter for converting, for example, the conventional 110 volt AC input line voltage to higher DC voltage levels.
The motor windings of induction motors have been excited by square waves and means have been provided for controlling the speed of induction motors driven in this manner. Since square wave excitation results in a winding current which varies inversely with frequency these prior art drive techniques have included means for varying, directly with frequency, the voltage impressed on the motor winding or windings.
One known technique for varying the excitation voltage comprises an impedance in series with the motor windings and the high voltage switches of the drive circuitry. One of the disadvantages is that the series impedance must be reduced to a negligible value to achieve high motor speeds and alternatively greatly limits the drive efficiency at lower motor speeds.
Another more elaborate approach to speed control comprises means for varying the DC supply voltage to the high voltage switches of the drive circuitry. This technique is not sufficiently sufficient in its operation. With this technique the efficiency is dependent upon the efficient conversion of line voltage AC to DC of a lower voltage of the line voltage peak value.
Accordingly, it is an object of the present invention to provide an improved apparatus for accurately controlling the speed of an induction motor over a relatively wide speed range.
Another object of the present invention is to provide a speed control apparatus which is characterized by a highly efficient power conversion.
Still another object of the present invention is to provide a speed control circuit for controlling an induction motor and which can be manufactued at a minimum cost.
A further object of the present invention is to provide an improved AC to CD converter which efficiently converts a conventional AC voltage into a DC driving voltage for the high voltage switches which in turn drive the motor winding or windings.
Still another object of the present invention is to provide a speed control technique which is usable with either a single phase motor or a multi-phase motor.